The role of aerosols and greenhouse gases in Sahel drought and recovery

Climatic Change - We exploit the multi-model ensemble produced by phase 5 of the Coupled Model Intercomparison Project (CMIP5) to synthesize current understanding of external forcing of Sahel...     

SELFE: A semi-implicit Eulerian–Lagrangian finite-element model for cross-scale ocean circulation

Unstructured-grid models grounded on semi-implicit, finite-volume, Eulerian–Lagrangian algorithms, such as UnTRIM and ELCIRC, have enjoyed considerable success recently in simulating 3D estuarine and coastal circulation. However, opportunities for improving the accuracy of this type of models were identified during extensive simulations of a tightly coupled estuary–plume–shelf system in the Columbia River system. Efforts to improve numerical accuracy resulted in SELFE, a new finite-element model for cross-scale ocean modeling. SELFE retains key benefits, including computational efficiency of existing semi-implicit Eulerian–Lagrangian finite-volume models, but relaxes restrictions on grids, uses higher-order shape functions for elevation, and enables superior flexibility in representing the bathymetry. Better representation of the bathymetry is enabled by a novel, “localized” vertical grid that resembles unstructured grids. At a particular horizontal location, SELFE uses either S coordinates or SZ coordinates, but the equations are consistently solved in Z space. SELFE also performs well relative to volume conservation and spurious oscillations, two problems that plague some finite-element models. This paper introduces SELFE as an open-source code available for community use and enhancement. The main focus here is on describing the formulation of the model and on showing results for a range of progressively demanding benchmark tests. While leaving details to separate publications, we also briefly illustrate the superior performance of SELFE over ELCIRC in a field application to the Columbia River estuary and plume.     

Annual Variation of Benthic Nutrient Fluxes in Shallow Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Abstract. Benthic fluxes of dissolved N. Si and P nutrients, alkalinity, dissolved inorganic C (DIC), and O₂ from sediments in the Gulf of Trieste (northern Adriatic, Italy) were measured monthly for 16 months, using laboratory incubated flux chambers at in siru temperatures in the dark. The annual average fluxes were: 02 = -19.3 ± 8.2, DIC = 13.7 ± 9.6, NO3 = -0.04 ± 0.16, NH4 = 0.3 ± 0.4. PO₄= 4.001 ± 0.01, Si = 0.9 ± 0.1 mmol m-² d^-1, with strong temporal fluctuations. The highest effluxes of all nutrients and DIC were observed in the summer. Small effluxes of DIC and NH4 and influxes of Si and PO4 were observed in late winter. Only NH₄ (ca. 50%) and Si (ca. 70%) fluxes were significantly correlated with temperature. This correlation suggests that the rate of downward input and the quality of sedimented organic matter (autochthonous and allochthonous) were superimposed on the temperature fluctuations. High DIC, NH₄ and Si effluxes observed in May 1993 during low temperature were due to the degradation of sedimentary organic matter produced by an early spring bloom of benthic microalgae which occurred about 6 weeks earlies while the autumn phytoplankton bloom was simultaneously reflected in enhanced benthic fluxes due to higher temperature. The role of benthic biological advection in this transport across the sediment-water interface, evaluated by comparison between measured benthic and calculated diffusive fluxes from nutrient pore water concentrations, was of minor importance. This is probably due to low infaunal activity throughout the year it was localized mostly in the narrow surficial layer. The annual average diffusive fluxes of NH4 and PO4 were higher than those measured, probably due to the presence of nitrificationdenitrifi-cation processes and redox-dependent chemical reactions at the oxic sediment-water interface, respectively. Only during bottom-water hypoxia in September 1993 did strong PO₄ effluxes prevail. Calculations based on the Redfield stoichiometry of oxic decomposition of organic N to NH4 and NO3, and differences between diffusive and measured NH₄ fluxes showed that denitrifkation averaged 0.8 mmol m^-2 d^-1. Significant correlations between NH₄ and PO₄ DIC and Si, and NH4 and Si fluxes suggested their parallel regeneration and utilization at the sediment-water interface. The nutrient fluxes observed were not significantly linked to O₂ consumption, suggesting also that anaerobic oxidation processes were important at the sediment-water interface in the gulf. The N, P and Si nutriqnts released from sediment pore waters are probably utilized in benthic microalgal and bottorn-hater primary production. This indicates that pelagic and benthic communities in the central part of the Gulf of Trieste function relatively independently of each other.     

Analytical study of the swing-by maneuver in an elliptical system

The objective of the present paper is to derive a set of analytical equations that describe a swing-by maneuver realized in a system of primaries that are in elliptical orbits. The goal is to calculate the variations of energy, velocity and angular momentum as a function of the usual basic parameters that describe the swing-by maneuver, as done before for the case of circular orbits. In elliptical orbits the velocity of the secondary body is no longer constant, as in the circular case, but it varies with the position of the secondary body in its orbit. As a consequence, the variations of energy, velocity and angular momentum become functions of the magnitude and the angle between the velocity vector of the secondary body and the line connecting the primaries. The “patched-conics” approach is used to obtain these equations. The configurations that result in maximum gains and losses of energy for the spacecraft are shown next, and a comparison between the results obtained using the analytical equations and numerical simulations are made to validate the method developed here.     

Nonequilibrium Processes in the Solar Corona,Transition Region,Flares, and Solar Wind <Emphasis Type="Italic">(Invited Review)</Emphasis>

We review the presence and signatures of the non-equilibrium processes, both non-Maxwellian distributions and non-equilibrium ionization, in the solar transition region, corona, solar wind, and flares. Basic properties of the non-Maxwellian distributions are described together with their influence on the heat flux as well as on the rates of individual collisional processes and the resulting optically thin synthetic spectra. Constraints on the presence of high-energy electrons from observations are reviewed, including positive detection of non-Maxwellian distributions in the solar corona, transition region, flares, and wind. Occurrence of non-equilibrium ionization is reviewed as well, especially in connection to hydrodynamic and generalized collisional-radiative modeling. Predicted spectroscopic signatures of non-equilibrium ionization depending on the assumed plasma conditions are summarized. Finally, we discuss the future remote-sensing instrumentation that can be used for the detection of these non-equilibrium phenomena in various spectral ranges.     

Morphologic variability of exposed mass-transport deposits on the eastern slope of Gela Basin (Sicily channel)

The NE portion of Gela Basin in the Sicily Channel is affected by multiple slope failures originated during the late‐Quaternary. Basin sequences show evidence of stacked acoustically transparent and/or chaotic units, characterized by irregular upper surfaces, interpreted as mass‐transport deposits. The seafloor morphology also shows evidence of both old, partially buried, as well as recent slide products. Two recent slides exposed at seafloor, only 6 km apart (Twin Slides), are similar in geomorphological parameters, age and multistage evolution. Multistage failure of Twin Slides evolved from mud flows, derived from the extensive failure of less consolidated post‐glacial units, to localized slides (second stage of failure) affecting older and more consolidated materials. Although Twin Slides are very close to each other and have similar runout and fall height, they produced very dissimilar organization of the displaced masses, likely reflecting the distinct source units affected by failures. Integrating geophysical, sedimentological, structural and palaeontological data, a detailed investigation was conducted to determine the size and internal geometry of this mass‐transport complex, to explain the differentiated product and to shed light on its predisposing factors, triggers and timing.     

Mortality by cause of death in Colombia: a local analysis using spatial econometrics

Journal of Geographical Systems - Colombia is undergoing major changes in mortality patterns. National- and department-level cause-specific analyses have previously been carried out, but very...     

A “Spring–mass” model of tethered satellite systems: properties of planar periodic motions

This paper is devoted to the dynamics in a central gravity field of two point masses connected by a massless tether (the so called “spring–mass” model of tethered satellite systems). Only the motions with straight strained tether are studied, while the case of “slack” tether is not considered. It is assumed that the distance between the point masses is substantially smaller than the distance between the system’s center of mass and the field center. This assumption allows us to treat the motion of the center of mass as an unperturbed Keplerian one, so to focus our study on attitude dynamics. A particular attention is given to the family of planar periodic motions in which the center of mass moves on an elliptic orbit, and the point masses never leave the orbital plane. If the eccentricity tends to zero, the corresponding family admits as a limit case the relative equilibrium in which the tether is elongated along the line joining the center of mass with the field center. We study the bifurcations and the stability of these planar periodic motions with respect to in-plane and out-of-plane perturbations. Our results show that the stable motions take place if the eccentricity of the orbit is sufficiently small.